Piezometer



J. A. FRIED PIEZOMETER March 3, 1942.

Filed Nm). 5o, 1959 Patented Mar. 3, 1942 l PmzoME'rEa Joseph A. Fried, Haddon Heights, N. J., assignor to Radio Corporation of tion of Delaware America, a corpora- Appiication November 30, 1939, Serial No. 306,859

7 Claims.

This invention relates to piezo-electric apparatus and particularly to apparatus for measuring instantaneous and/ or continuously variable forces by utilization of the piezo-electric properties characteristic of certain crystalline substances, such, for example, as quartz, tourmaline, Rochelle salt, etc.

Piezometers employing one or more piezo-electric crystals have heretofore found useful application in the study of pressure, and time-pressure phenomena. The prior art as to this is well exemplified in U. S. Patent No. 2,096,826 to Schrader and in U. S. Patent No. 1,930,905 to Nicolson.

The Schrader piezometer, when used in connection with suitable indicating means (such, for example, as a cathode ray oscillograph), is well suited for the study of forces of a single sign (positive) but is not suitable for producing an equally accurate indication of a force or forces of both positive and negative signs. By way of example: the Schrader piezometer is designed to accurately indicate the intensity and duration of an explosion, such as one resulting from the ignition of a vapor charge in the cylinder of an internal combustion engine, but does not produce an equally accurate indication of the conditions obtaining in that cylinder when the pressure therein is less than atmospheric pressure. It thus may be said to be adapted to produce a unilateral indication, only.

The piezometer oi.' the Nicolson patent employs an oscillating crystal the frequency of which is altered by variations in the clamping force applied to the said crystal by the device under test. Since an increase or decrease in the force applied to the crystal is indicated by a corresponding change in its frequency of oscillation a bilateral indication is achieved. However, the extent or range of the indications produced is limited not by the elastic limits of the piezo-electric crystal employed but by the very much more limited ability of the crystal to sustain oscillation when subject to a clamping force. n

Accordingly, the principal object of the present invention is to provide a piezometer capable of producing an accurate indication of reciprocating or bilateral forces and one wherein the intensity of the forces applied thereto and the accuracy of the indication produced thereby are limited substantially only by the elastic limits of the material constituting the piezo-sensitive elements of the device.

Another object of the invention is to provide an metal.

accurate, simple, inexpensive and trouble-free piezometer employing a nonfoscillating quartz crystal and one wherein bilateral sensitivity is achieved without the use of springs or similar elastic auxiliary loading elements. o

Other objects and advantages will be apparent and the invention itself will be best understood by reference to the following specification and to the accompanying drawing, wherein:

Figure l is a side elevational view, with one Wall of the housing removed, of a, piezometer constructed in accordance with the principle of the invention,

Figure 2 is a sectional view taken on the line 2--2 of Fig. l, and

Figure 3 is an electrical diagram showing a simplified view of the instrument of Fig. 1 in circuit with an amplifier and cathode-ray oscilloscope.

In the drawing wherein like reference characters represent the same or corresponding parts in all figures, I. designates generally a housing or box comprising a base 2, a pair of end walls 3 and 4, a pair of side walls 5 andv and a lid or cover 1. The box I is preferably of rugged construction and may be constituted in its entirety of Secured to the center of the base as by welding or otherwise is a vertical arm or contact member 9 which is constituted of steel or other relatively flexibleA metal and which may be of reduced cross section adjacent its base, as indicated at 8a, whereby its flexibility at that area is enhanced. The arm 8 extends through an orifice la provided for the purpose in the cover plate 'I and is preferably provided at its free end with a longitudinal extension 9 which may terminate in a bifurcated or other suitably shaped part 9a capable of receiving a vibrating rod, shaft or other part I0 which will be understood to comprise, or to be coupled to, the machine or other device whose vibrations or other forces are to be studied. Obviously, any eccentric movement of the coupling or shaft I0 will serve to flex the contact arm 8 in the opposite directions indicated by the arrows II.

Movement of the arm 8 to the left` as viewed in all figures, results in the application of a compression force to a piezo-electric unit which may comprise a pair of piezoelectric elements I2, I4, an interposed electrode I6, and a pair of crystal supporting members I8 and 20, respectively. The supporting member I8 may comprise a short rodlike member I 8a, one end of which abuts the lever arm 8 and the other end of which terminates in a recessed head Ib within which the crystal I2 is seated. The support 20 for the other crystal of the pair may comprise a threaded bolt 20a which extends through the end-wall 3 of the housing and which may be secured against undesired movement with respect thereto by means of a nut 20c which abuts the outer surface of this wall. The bolt 20a, like the rod-like support Ilia, is preferably provided at its inner end with a recessed head 20h in which one of the crystals, in this case the crystal I4, is seated.

A preferably duplicate piezo-electric unit comprising a pair of piezo-electric elements I3 and I5. an interposed electrode I1, a supporting rod I9, and a clamping bolt 2I, is mounted within the housing I at the opposite side of the flexible lever or contact arm 8.

The piezo-electric elements I2, I4, and I3, I are each preferably constituted of quartz or of tourmaline, though they may comprise Rochelle salt crystals in cases where the intensity of the forces to be studied is not such as to require the use of the above mentioned physically stronger piezo-electric materials. In any event, these crystals, or at least one crystal of each pair, are so cut with respect to their natural crystalline axes (they may be X-cut) and are so mounted with respect to the interposed electrodes, I6 and I1, respectively, that when subject to compression forces of equal intensity, the electrode faces of the crystals contiguous one electrode (say, electrode I6) are of the positive sign and the electrode faces of the crystals contiguous the other electrode (say, electrode I1) are of the negative sign. When the supporting bolts I9 and 20 are properly adjusted, the compression forces applied to the crystals will ordinarily be of equal intensity when the lever or tlexible contact arm 8 is in its untilted or unilexed position.

As shown more clearly in Figs. 2 and 3, the electrodes I6 and I1 for the oppositely polarized oppositely located crystals I2, I4, I3, I5 are electrically connected by a common lead 22 which extends through an insulating bushing 23 in the side wall S of the housing. The return or ground connection to the crystals is through the metal supports I8-2I.

When the crystals are polarized and mounted in the manner above described, it will be apparent that when the contact arm 8 is moved or flexed to the left, it is brought selectively into 'operative relation with the crystal or crystals which are to the left of it. Thus, the mechanical force applied to the crystals I2 and I4 is increased and the clamping pressure on the crystals I3 and I5 is relaxed. The increase in pressure upon the crystals I2 and Il results in an increase in the positive charge normally transferred to the electrode I6. The decrease in pressure upon the crystals I3 and I5 reverses the (negative) polarity of the charge normally transferred therefrom to the electrode I1. Since the electrodes II, I1 have a common lead 22, the now electrically positive charge or voltage upon electrode I1 is added to that appearing upon electrode I6; hence the voltage in lead 22 is increased and the sensitivity of the device is augmented. When the contact arm 8 is moved or flexed to the right. the mechanical and hence the electrical charge appearing in the conductive connection 22 is reversed, that is, in this case, the sign of the voltage in that lead becomes negative Obviously, in either case, the voltage flowing to ground through the metal supports is equal and opposite to that present in the electrode lead 22.

The sensitivity of the device may be further increased by the addition of more piezo-electric the other hand, at some sacrifice in sensitivity,`

one of the crystals in each pile. say the piezoelectric crystals I4 and I5, may be omitted providing that suitable insulating mounting means are employed for preventing the charge on theA remaining crystals from leaking to ground. The problem of mounting two discrete crystals, however, is not as simple as it might at first glance appear since the mount in addition to its insulating properties must be highly elastic; that is to say, it must be capable of being deformed and restored at a rate corresponding to the rate of deformation of the crystals themselves. A mount formed at' least in part of a so-called Z-cut quartz crystal (i. e., one having little or no piezo-electric properties) would possess the requisite insulating and elastic properties.

In view of the foregoing, it is apparent that when at least two piezo-electric crystals are mounted on each side of the contact arm 8, in the manner above described, one crystal of each pair serves perfectly (because of its insulating properties) to prevent electrical leakage and is further usefully employed (by reason of its own piezoelectric properties) to increase the intensity of the electrical charge generated by the crystal adjacent thereto.

Referring to Fig. 3 which shows one manner in which the piezometer of Figs. l and 2 may be set up in adapting it to one of the numerous uses of which it is capable. In this schematic diagram it may be assumed that the rod or shaft III whichl is fitted within the bifurcated extension 8 of the contact arm l comprises the connecting arm of a U-shaped element or cradle whose upstanding arms `are mechanically coupled to the opposite bearings of a rotor shaft for a motor or the like (not shown) which is to be examined in order to correct for any unbalance which may exist in the said shaft. In this case, where a single piezometer is employed, the bifurcated arm 8 may first be coupled to that end of the cross bar III of the cradle which is adjacent one bearing of the shaft under test. Now, if the common lead 22 for the electrodes I8 and I1 of the piezometer are connected as though an amplifier 25 to one pair of deecting plates (say the vertical plates) 26 of a cathode ray oscilloscope 21 whose ray or beam 28 is normally directed across thel screen 29 in a predetermined path (say a straight line) under control of a local oscillator 30 (whose frequency is preferably proportional to the speed of the shaft under test) then the voltage on the vertical deflecting plates 26 will cause the trace of the beam to depart from its normal pattern. Thus, if the voltage derived from the local oscillator 30 and applied to the other (horizontal) deflectlng plates II is such as to cause the beam to normally trace a straight line upon the screen 29, then the presence of a voltage upon the plates 28 will be evidenced by a departure from straightness. The location of the irregularity in the pattern, i. e., whether the irregularity is above the line or below it, will indicate to the observer that the unbalance in the shaft under test is localized in one of the cross-sectional halves of the shaft. This half section may then be weighted temporarily (as with a pellet of clay of a weight sufficient to correct the unbalance). The piezometer may then be moved along the coupling l0 to a position adjacent the other bearing of the shaft under test and the test repeated. Obviously, two piezometers may be employed if desired to produce simultaneous indications of the condition of balance (or a comparative indication of any difference in torsional forces) adjacent the opposite ends of the shaft or other device under test.

In conclusion, it may be pointed out that where, as in the preferred embodiment of the invention, there are two crystals in each pile and the outputs of the two piles are connected` to a common lead 22, any difference in the sensitivity (i. e., piezoelectric properties) in the crystals of a given pile will be relatively the same irrespective of the direction in which the actuating y.force (applied through arm 8) is applied.

Various modifications of the invention will suggest themselves to those skilled in the art. It is to be understood, therefore, that the foregoing is to be interpreted as illustrative and not in a limiting sense except as required by the prior art and by the spirit of the appended claims.

What is claimed is:

1. A pair of piezo-electric elementsl having oppositely located oppositely polarized faces, a.

movable contact member mounted between said oppositely polarized opposed faces, means for maintaining said elements under substantially equal pressure when said movable contact member is idle and means responsive to movement of said contact member in a given direction for increasing the pressure on one Iof said piezo.

` sponding substantially to comitantly inversely varying the pressureV upon said piezo-electric elements whereby to increase the intensity of the piezo-electric charge appearing upon one polarized face of one of said elements and to change the sign of the piezoelectric charge appearing upon the other polarized face of said other element.

3. The invention as set forth in claim 2 and wherein the oppositely polarized faces of said piezo-electric elements are electrically connected.

4. In combination, a pair of piezo-electric crystal elements mounted in pressure transfer relation and having oppositely located, opposite- 1y polarized electrode faces, and pivotal means interposed between said electrode faces for concomitantly inversely varying the pressure upon said crystal elements.

5. In combination, a pair of piezo-electric crystal elements having oppositely located, oppositely polarized faces, and means including a lever interposed between said oppositely polarized faces for mounting said crystal in variable pressure transfer relation.

6. A piezometer comprising a base, a lever pivotally supported on said base, a pair of piezoelectric units mounted in pressure transfer relation with respect to and on opposite sides of said lever, each of said units comprising a pair of quartz crystal elements and anl interposedr electrode, the electrode faces of the' crystals ofv one unit being oppositely polarized with respect to the corresponding electrode faces of the other unit,and an electrical connection between the said electrodes of said piezo-electric units.

7. The invention as set forth in claim 2 and wherein said mounting means comprises a medium having a modulus of elasticity correthat of said piezoelectric elements. g

J'IOSEPH A. FRIED. 

